Field of the Invention
The present invention relates generally to an earmuff for protecting the ears of a person from noise. In particular the invention is directed to earmuffs of an improved type to provide for additional attenuation of external sound.
In many types of vocations it is necessary for the personnel to be exposed to loud and piercing noises for periods of time. It is therefore desirable to provide for suitable ear protection so as to protect the personnel from any serious or permanent damage to the eardrum.
One type of ear protection that is currently in use are earmuffs, which are comfortable to wear and yet provide suitable protection from ambient noise levels. Typically the earmuffs currently in use are made of a hard outer plastic shell lined with an inner foam cup and with an outer cushion to seal the earmuff around the ear of the user.
In all the various components used to form the earmuff it is desirable to provide for the maximum attenuation consistent with cost and weight.
In the present invention an earmuff is designed so as to provide for additional attenuation compared to earmuffs of similar design. Additionally, the higher attenuation is achieved using relatively inexpensive materials.
In the prior art the general belief is that in order to achieve a high attenuation the earmuff must have high mass. Because of this, prior art earmuffs have used fairly thick walls for the outer shell or have used higher specific gravity materials in their construction. For example, prior art earmuffs can use a higher price, high specific gravity plastic such as ABS plastic in place of inexpensive polypropylene. In the present invention the earmuff provides for excellent attenuation characteristics with the less expensive plastic polypropylene and also with the use of thinner walls and thereby less plastic material then would generally be used in the prior art.
In the present invention the additional attenuation in the outer cup is achieved by raising the specific gravity of the cup as opposed to making the walls heavier. This results in improved molding and less material. The difficulty with using thicker walls or thicker parts in plastic has a lot to do with the molding of these parts. The thicker the walls the longer the cycle time and the more the molded part can distort. Also the more the part can shrink. In addition, the use of more plastic can significantly increase the cost of the part. The present invention uses a thinner cup wall and increases the specific gravity by including a filler in the plastic.
Unfortunately the use of the standard fillers in increasingly larger amounts does not increase the attenuation, but rather provides for less attenuation. It appears that the addition of the filler makes the cup stiffer as well as heavier. The additional stiffness decreases the attenuation and therefore the use of standard fillers in order to add mass does not achieve the desired results.
The present invention therefore uses a particular type of filler which increases the mass significantly without increasing the stiffness. In addition, the material used as filler is inexpensive in cost and is easy to process in the plastic during the molding. The present invention uses an iron powder incorporated in the inexpensive plastic such as polypropylene. This achieves the desired increase in mass at a relatively low cost without the increase in stiffness.
In particular, a specific Carbonyl iron powder is used to provide for the desired filler material to the plastic. The testing of attenuation for the earmuff using the Carbonyl iron powder included in the plastic cup provided for an increase in attenuation when compared to thicker cups without the iron powder filler. It was therefore determined that the iron powder worked quite well in bringing the attenuation numbers up to a desired level.
It should be appreciated that other types of metal powders could also be used. Iron, however, because it is inexpensive in cost and is inert in use and because it incorporates itself well within the polypropylene, appears to be particularly desirable in use.
A clearer understanding of the invention will be had with reference to the following description and drawings wherein.